Using ROS for a Delta Robot

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Hi,

This is question for Steve: I'll be in a near future interested in simulating delta robots in Gazebo and control it with ROS.

We have analytical direct and inverse kinematics and dynamics. We actually already build and control our real prototypes, but we're doing that based on matlab, and I want to port this on ROS...

I understand from your post that you already implemented such solutions for delta robots.

Could you explain further how you did it and maybe share some code?

Thanks in advance!

Damien

My solution for implementing a closed kinematic chain manipulator (in my case a five bar mechanism) was to write the URDF as two serial manipulators. I "close" them myself with my own fwd/inv kinematics equations. In other words, there are four joints, but only two DOFs, and so while the left manipulator has j1 & j3, and the right manipulator has j2 & j4, we can write two of the four joints in terms of the other two, and only really care about two of the joints for control. You publish the joint angles of all four joints, and rviz, etc, are all happy.

Steve.

Thanks for your help :)

You wrote:

(a) add URDF support for parallel robots and give us a patch we can then integrate. This is going to be a little complicated though.

We think this is the best way. Could you elaborate on the complications?

We don't know for sure if we choose this path, because option b) or our own option c) (creating a new package) seem plausible as well.

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We probably first try option b, because it seems easier than creating a new package or adding support for a parallel robot ROS wide. I come back when i have any more questions :-) Thanks!

Thanks

While URDF does not support non-tree kinematic structures at this point, gazebo can handle closed chain loop dynamics. Maybe as a first order hack for simulating the Delta Robot, you can leave some joints out of the URDF, keeping the URDF representation in a tree structure, but in addition implement the loop closing joints in gazebo only. As an example, I did this for the PR2 gripper, where some of the joints in simulation are not modeled in the corresponding URDF. Is this is a feasible approach for your model?

Hello,

Thanks for your answer. we did discover a new problem, however. After reading a bit more i found the following text:

'The Unified Robot Description Format (URDF) is an XML specification to describe a robot. We attempt to keep this specification as general as possible, but obviously the specification cannot describe all robots. The main limitation at this point is that only tree structures can be represented, ruling out all parallel robots.'

And a Delta Robot is a parallel robot. Now we are wondering on what to do. We could rewrite arm_kinematics and rename it to parallel_kinematics or something like that, but perhaps some of you have other/better ideas than we have :-)

If you implement the functions mentioned in the article you will have a working version of the higher level functionality for motion planning and kinematics. You still have to implement your own lower-level controllers to talk to your robot motors, etc before you can actually get this working on the robot. You might want to look at simulating your robot in Gazebo and trying out the higher-level functionality there while developing the lower level controllers for the robot in parallel.